Advances in light-emitting diodes (LEDs) have made light sources constructed from such devices attractive alternatives to conventional light sources such as fluorescent lights and incandescent lights. LED-based light sources have energy conversion efficiencies approaching or exceeding those of these conventional light sources. In addition, the LED-based light sources have lifetimes that far exceed those of these conventional light sources. For example, a fluorescent light source has a lifetime of about 10,000 hours whereas an LED has a lifetime of 100,000 hours. In addition, fluorescent light sources tend to fail completely without warning. In contrast, LED-based light sources tend to fade, and hence, the user has adequate warning before it completely fails.
Unfortunately, LEDs have some drawbacks when used to replace conventional light sources. First, LEDs emit light in relatively narrow spectral bands, and hence, to provide a source that is perceived to have an arbitrary color, a number of LEDs that emit different narrow spectral lines must be packaged together or the LED must be covered with one or more phosphors that are excited by the LED to provide the desired output spectrum.
In addition, a single LED has only a limited light output. Even high-power LEDs are limited to a few watts at best. In addition, as noted above, to provide an arbitrary output spectrum a number of LEDs must be combined into a single light-emitting element. Hence, to provide a light source having an output of more than a few watts in any spectral band, a number of LEDs must be combined into a single unit.
To provide such multi-LED devices, a number of LED dies are normally connected to some form of substrate. The connection schemes can be divided into two broad categories. The first category relies on wire bonds to connect one or more of the electrodes on the die to corresponding electrodes on the substrate. These schemes have a number of problems. First, the wire bonds must be individually applied. Second, the wire bonds must be protected due to their fragile nature. The protection typically involves potting the die and wire bonds in some clear encapsulant. Unfortunately, the encapsulant can age causing light absorption. In addition, the encapsulant stresses the wire bond, which can lead to premature device failure. Further, the encapsulation is an additional assembly step that results in additional cost. Moreover, the encapsulant often imposes maximum temperature constraints on the device. In addition, encapsulants can stress the LED materials causing increases in the required operating voltage. Third, the wire bonds typically block part of the light emitted by the LEDs, and hence, reduce the efficiency of the light source. Finally, it should be noted that wire bond failures are a significant source of the overall device failures.
The second category of connection schemes, in principle, avoids the wire bonds, and hence, the problems associated with those bonds. These schemes are commonly referred to as flip-chip schemes. In these schemes, the LED is fabricated on a transparent substrate by depositing a number of layers on the substrate. Since the layers needed to construct an LED are well known in the art, these layers will not be discussed in detail here. For the purposes of the present discussion, it is sufficient to note that an LED has three major layers, an n-type layer that is usually deposited first on the substrate, an active layer that generates the light, and a p-type layer. Electrons flow from the n-type layer into the active region where the electrons combine with holes that flow into the active layer from the p-type layer.
To power the LED, a potential must be provided between the n-type layer and the p-type layer. But the n-type layer is buried within the multi-layer stack. There are two basic configurations used to deal with this connection problem. In the first configuration, the n-type and p-type connections are made through electrodes that reside on the outer surfaces of these layers. This type of device will be referred to as a “vertical device” in the following discussion. The second type of device is referred to as a “lateral device”. In a lateral device, the connection to the buried layer is provided by etching the layer above it to expose the buried layer. In the example discussed above, the p-type layer and the active layer are removed in part of the device to expose the underlying n-type layer. A connection is then provided to the p-type layer by depositing a metal film on the exposed layer. In a device that is connected via wire bonds, one of the wire bonds is affixed to this metal film. The device is referred to as a lateral device because the current must flow laterally from the exposed mesa to reach the active layer. A flip-chip LED is an example of such a lateral device.
In a flip-chip LED, the connection to the n-type layer is provided by etching the p-type and active layers in one area of the device to expose the n-type layer. A conducting layer is then deposited on the exposed n-type layer mesa and used to make the connection to that layer. To mount the chip on a carrier such as a printed circuit board, the chip is turned upside down such that the contacts on the top of the LED will mate with pads on the printed circuit board. The chip is then bonded to the printed circuit board.
While flip-chip LEDs avoid the problems associated with wire bonding, they introduce a new set of problems. First, the chips must be placed on a printed circuit board, or the like. The precision required in this operation is high, since the contacts are very small and close together. The final product manufacturer may not have the equipment needed to make this type of precise placement in an economical manner. Hence, these devices are often packaged on a separated carrier that is analogous to a small printed circuit board that has pads that are further apart, and hence, reduces the precision with which the final product manufacturer must place the device. Unfortunately, this solution increases the size of the packaged device, and hence, limits the density of LEDs in the final product. In addition, this solution increases the cost of the final LED, since the LEDs must be individually connected to the carrier.
Second, the bonding of the flip chip to the carrier, whether the final printed circuit board or the above-described intermediate carrier, involves processing steps that can lead to shorts between the layers of the LED. These shorts can occur at the time of bonding or during the life of the device. The shorts increase the cost of the LEDs by reducing yields.
Third, the mesa that is cut to provide the n-type contact occupies a significant fraction of the surface area of the die. This area does not produce light, since the mesa must be cut through the active layer. Hence, the total light per unit area leaving the device is significantly reduced.